Hydroprocessing operations are widely used in the petroleum refining industry for a number of purposes. Hydrotreating and hydrofinishing process, for example, are generally used to reduce the impurity levels of petroleum oils, mostly by the removal of organic heteroatoms, especially sulfur and nitrogen, by their conversion to inorganic form permitting their removal from the oil. Hydrocracking is, of course, an established process in petroleum refining for the production of gasoline and distillate products from heavy oils and high boiling fractions from other processes including the refractory aromatic materials obtained from catalytic cracking. Hydrodemetallation used for reducing the metal content of oils, particularly reduced crudes and residual materials. The catalysts used in these processes are usually bifunctional materials which possess acidic functionality coupled with hydrogenation-dehydrogenation activity provided by a metal component which is usually selected from Group VIII of the Periodic Table, often combined with a Group VI metal in the case of Group VIII base metals, for example, in combinations such as Ni-W, Co-Mo, Ni-Mo. The acidic activity of the catalyst will vary according to the type of operation with hydrocracking catalysts using relatively acidic catalysts to achieve the required boiling range conversion which is desired. Hydrotreating, hydrofinishing and hydrodemetallation ctalysts, by contrast, may suffice with lower inherent acidities. The acidic activity is ususally provided by the porous support material for the metal component and for this purpose, oxide type materials are conventionally used, especially oxides such as alumina and silica-alumina.
The use of zeolite based hydrocracking catalysts is progressively increasing because of the higher activity and long term stability of these catalysts and for this purpose, large pour size zeolites such as zeolite X or the various forms of zeolite Y such as ultrastable zeolite Y (USY) are becoming conventional.
A notable advance in hydroprocessing technology is disclosed in U S. Pat. No. 4,419,220 (LaPierre) which discloses a process for hydroisomerizing paraffins using a catalyst based on zeolite beta. A related process is disclosed in EP 94827, in which zeolite beta is used as the catalyst for a hydrocracking operation which is capable of producing low pour point distillate products from high boiling feeds. Zeolite beta is believed to be unique in its capability of effecting a simultaneous reduction of pour point and boiling range by reason of its ability to selectively isomerize and convert paraffins in the presence of aromatics. Large pore size zeolites such as zeolites X and Y are aromatic-selective in contrast to zeolite beta's paraffin-selective behavior.
In hydroprocessing operations, hydrogen is consumed as a consequence of aromatics saturation, ring opening and cracking reactions as well as by removal of heteroatoms following ring opening. Hydrogen consumption tends to increase with conversion (defined as the increase in the amount of fractions boiling below a certain temperature, expressed as a weight percentage of the feed). During hydrocracking, the catalyst operates in two distinct phases during each hydrocracking cycle. Initially, the catalyst ages relatively rapidly from start of cycle (SOC) until lineout is obtained. Once lineout is attained, the catalyst is in relative equilibrium under a given set of feed constraints and reaction conditions and the rate of aging decreases materially until the end of cycle (EOC) is reached. During the start up phase, prior to attainment of lineout, reactor temperature is increased relatively rapidly, typically at a rate from 2.degree. to 10.degree. F./day, in order to maintain conversion relatively constant as the catalyst ages at its initial fast aging rate. After lineout is attained, the reactor operating temperature is increased at a rate of usually no more than 0.2.degree. .F/day consequent upon the lower catalyst aging rate during this phase of the operation. Finally an end of cycle temperature is reached, at which the activity and/or selectivity of the catalyst has decreased to an unacceptable level and to this point reactivation or regeneration of the catalyst is carried out to restore activity and selectivity.
The activity of the catalyst in hydrocracking may be controlled by the presence of selective catalyst poisons in the feed which interact with the acidic sites on the bifunctional hydrocracking catalyst so as to reduce acidic activity while maintaining a relatively constant level in the hydrogenation function provided by the metal component of the catalyst. Processes in which nitrogenous compounds such as ammonia or organic amines are introduced into the hydrocracking zone together with the feed are disclosed, for example, in U.S. Pat. Nos. 3,524,807 (Lewis), 3,657,110 (Hengstebeck) and 3,186,296 (Hass). The effect of the nitrogen additions may vary according to the characteristics of the catalyst with certain highly siliceous catalysts being more resistant to the effects of nitrogen than others, as disclosed in U.S. Pat. No. 4,441,991 (Dwyer). GB 1 429 291 discloses a lube hydrocracking process in which various nitrogenous compounds may be added to the feed in order to maintain a relatively constant level of cracking activity as feeds with different nitrogen content are utilized in the process.
Hydrogen consumption is an important operating factor in a hydrocracking unit since hydrogen is relatively expensive to produce; unnecessary consumption should be minimized as far as possible. The hydrogenation reactions which take place during hydrocracking are characteristically exothermic and are therefore thermodynamically favored by lower temperatures. For this reason, hydrocracking is conventionally carried out at temperatures which do not usually exceed 850.degree. F. at which point exothermic hydrogenation becomes thermodynamically less favored than endothermic cracking. At start up, however, relatively lower temperatures are employed because the catalyst is at its highest relative activity during this part of the cycle. These low temperatures are conducive to hydrogenation and accordingly the consumption of hydrogen during this part of the cycle is relatively high: the operation consumes more hydrogen for a given level of feed conversion, as compared to the consumption after lineout when the catalyst is essentially at equilibrium.